Dielectric-on-semiconductor devices comprise ferroelectric or piezoelectric material laminated to a semiconductor material. The mechanism of these devices involves the interaction between transport carriers in the semiconductor material and polarization changes in the dielectric material induced by electric fields and/or physical stresses. Illustrative of such devices are ferroelectric field-effect transistors and amplifying acoustic surface wave transducers. See, e.g., U.S. Pat. Nos. 2,791,758, 2,791,759, 2,791,760, 2,791,761 and 3,832,700; Yamanishi, Kawamura and Nakayama, Appl. Phys. Lett. 21, 146 (1972); Collins, Lakin, Quate and Shaw, Appl. Phys. Lett. 13, 314 (1968); Lakin, Collins and Hogan, Proc. IEEE (Letters) 57, 740 (1949); and U.S. Pat. No. 3,828,283.
Ferroelectric field-effect transistors were previously made by laminating a bulk ferroelectric crystal to a separately made single-crystal semiconductor wafer. The ferroelectric material was a crystal of guanidinium aluminum sulfate hexahydrate (GASH), and the semiconductor material was a crystal of germanium. The air gap between the two materials was minimized by carefully lapping, polishing and cleaning the surfaces of the crystals in very flat, planar configurations, and in some devices, by filling the remaining air gap with a dielectric material such as ethylene cyanide or nitrobenzene. These devices were not, however, commercially successful. Poor efficiency was encountered in modulating the conductivity of the semiconductor surface with electrostatic charges induced by polarization of the ferroelectric material.
Ferroelectric field-effect transistors have also been made by deposition of polycrystalline or amorphous semiconductor material on a single-crystal or polycrystalline ferroelectric substrate. Specifically, a thin film of tellurium, cadmium sulfide or tin-doped indium oxide was vacuum evaporated on single-crystal triglycine sulfate (TGS) and barium titanate (BaTiO.sub.3) substrates, and on polycrystalline lead zirconate titanate (PZT) substrates. These transistors have not, however, been satisfactory because of the poor electrical instability of the semiconductor films. Specifically, the transconductance was low, and the gate threshold or gate turn-on and cut-off voltages would drift and decay the order of volts during operating periods of a few hours to a few days.
Ferroelectric field-effect transistors have also been made by deposition of a ferroelectric material on a bulk semiconductor crystal. See U.S. Pat. application Ser. No. 354,022, filed Apr. 24, 1973 by one of the coapplicants hereof, assigned to the same assignee as the present application. Specifically, a thin film of bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12) was deposited on a single-crystal silicon wafer by sputtering, vacuum evaporation, chemical vaporization, or spinning deposition techniques. This structure has certain advantages as described in the identified copending application, e.g., high transconductance and electrical stability in the "ON" and the "OFF" states. However, this structure was limited in modulation efficiency by reason of the properties of the ferroelectric film, which are inferior to the properties of corresponding bulk ferroelectric materials.
Similarly, amplifying surface wave transducers have been made previously by deposition of a thin film of piezoelectric material on a semiconductor substrate. The piezoelectric material is typically a polycrystalline zinc oxide, zinc sulfide or cadmium sulfide, and the semiconductor material is a single-crystal silicon. Interdigitated grid electrodes are positioned at opposite ends of the piezoelectric film to transmit and receive acoustic surface waves along the surface of the piezoelectric film, and localized high concentration impurity is provided in the semiconductor substrate adjoining the piezoelectric film at the interdigitated electrodes. A plane of conductivity is thus formed in the semiconductor at the piezoelectric film when an electric signal is applied to the electrode, which increases the capacitance and normal electric field component of the transducer, and in turn, amplifies the acoustic surface wave propagated and received. Such amplifying surface wave transducers are, however, limited by the properties of the piezoelectric thin film, which are generally inferior to the bulk counterparts.
The present invention overcomes these difficulties and disadvantages, and provides a dielectric-on-semiconductor device with increased modulation efficiency and a higher coupling constant than corresponding prior art devices. Further, it provides dielectric-on-semiconductor devices possessing the characteristics of both bulk semiconductor material and bulk ferroelectric or piezoelectric material.